The present invention relates to a steering device for vehicle, which utilizes a so-called steer by electric wire system.
In a vehicle steering device that employs a steer by electric wire system, the movement of a steering actuator, which corresponds to the operation of an operating member modeled on a steering wheel, is transmitted to the wheels of the vehicle in such a manner that the steering angle changes without this operating member being coupled mechanically to the wheels. In a vehicle that employs a steer by electric wire system such as this, a proposal has been made for computing a target yaw rate corresponding to the amount of operation of the operating member, and controlling the steering actuator such that the target yaw rate coincides with the actual yaw rate so as to stabilize the behavior of the vehicle.
FIG. 13 shows an example of a control block diagram of a vehicle steering device employing a conventional steer by electric wire system.
In the control block diagram, K1 is the gain of a target yaw rate xcex3* relative to the operating angle xcex4h of an operating member 101, and a steering device computes a target yaw rate xcex3* from the stored relationship of xcex3*=K1xc2x7xcex4h, and an operating angle xcex4h detected by a sensor. K2 is the gain of a target steering angle xcex4* relative to the deviation between the target yaw rate xcex3* and the actual yaw rate xcex3 of a vehicle 100, and a steering device computes a target steering angle xcex4* from the stored relationship of xcex4*=K2xc2x7(xcex3*xe2x88x92xcex3), the computed target yaw rate xcex3*, and a yaw rate xcex3 detected by a sensor. The gain K2 is regarded as a function of velocity V, and is set such that it decreases in line with an increase in velocity V in order to ensure stability at high speeds. Ga is the transfer function of the target drive current Ia* of the steering actuator 102 relative to the deviation between a target steering angle xcex4* and the actual steering angle xcex4 of the vehicle, and the steering device computes a target drive current Ia* from the stored relationship of Ia*=Gaxc2x7(xcex4*xe2x88x92xcex4), the computed target steering angle xcex4*, and a steering angle xcex4 detected by a sensor. The transfer function Ga is set, for example, such that proportional integral (PI) control is performed. K3 is the gain of a target operating torque Th* relative to the operating angle xcex4h of operating member 101, and the steering device computes a target operating torque Th* from the stored relationship of Th*=K3xc2x7xcex4h and an operating angle xcex4h detected by a sensor. Gb is the transfer function of the target drive current Ib* of the operating actuator 103 relative to the deviation between the target operating torque Th* and the actual operating torque Th, and the steering device computes a target drive current Ib* from the stored relationship Ib*=Gbxc2x7(Th*xe2x88x92Th), the computed target operating torque Th* and an operating torque Th detected by a sensor. The transfer function Gb is set, for example, such that proportional integral (PI) control is performed.
In the above-mentioned conventional constitution, because the actual yaw rate xcex3 of a vehicle does not increase when the coefficient of friction between the surface of a road and the tires is reduced by surface icing, or when tire lateral force reaches its limit, a saturated state results in which the yaw rate xcex3 does not attain the target yaw rate xcex3* when operating torque Th increases, and there is a possibility of the steering angle xcex4 diverging, and of vehicle behavior becoming unstable.
That is, FIG. 14 (1) and FIG. 14 (2) is one example of simulation results in a steering device constituting the above-mentioned conventional steer by electric wire system, showing changes over time in the yaw rate xcex3, target yaw rate xcex3* and steering angle xcex4 relative to a step input of 2.7 Nxc2x7m operating torque Th at times t1 to t2 (0.5 to 5 seconds), in a vehicle travelling at a velocity of 60 km/hour, wherein the coefficient of friction between the vehicle and the surface of the road is regarded as 1 up until t3 (2.5 seconds), and is regarded as 0.1 thereafter. The fact that the deviation between the yaw rate xcex3 and the target yaw rate xcex3* increases, and the steering angle xcex4 diverges in accordance with the drop in the coefficient of friction is shown.
Further, FIG. 15 (1) and FIG. 15 (2) depict Bode diagrams showing an example of yaw rate xcex3 frequency response simulation relative to operating torque input in a steering device constituting the above-mentioned conventional steer by electric wire system, wherein a vehicle is travelling at a velocity of 20 km/hour. Further, FIG. 15 (3) and FIG. 15 (4) depict Bode diagrams showing an example of yaw rate xcex3 frequency response simulation relative to operating torque input in a conventional steering device in which a steering wheel is mechanically coupled to the vehicle wheels, wherein a vehicle is travelling at a velocity of 20 km/hour. FIG. 15 (1) through FIG. 15 (4) indicate that, at low travelling velocity, yaw rate responsiveness relative to operating torque input decreases more in a vehicle steering device employing a conventional steer by electric wire system than in a steering device in which a steering wheel is mechanically coupled to the vehicle wheels.
An object of the present invention is to provide a vehicle steering device capable of solving the above-mentioned problem.
A steering device for vehicle of the present invention comprises an operating member operated by being rotated; a steering actuator driven in accordance with the operation of the operating member; means for transmitting the movement of the steering actuator to wheels of the vehicle such that the steering angle changes in accordance with the movement without mechanically coupling the operating member to the wheels; an operating actuator for generating control torque, which acts on the operating member; means for determining a load torque, which is sum of the control torque and the operating torque exerted on the operating member by a driver; means for determining the operating angle of the operating member which is operated by the action of the load torque; means for computing a target behavior index value of the vehicle, comprising at least a target yaw rate corresponding to the determined load torque and operating angle based on a stored relationship between the load torque, operating angle, and target behavior index value; means for determining a value, comprising at least the yaw rate of the vehicle, as a behavior index value corresponding to change of behavior of the vehicle; means for controlling the steering actuator such that the determined behavior index value follows the target behavior index value; means for computing a target operating angle of the operating member corresponding to the determined behavior index value, based on a stored relationship between the behavior index value and the target operating angle; and means for controlling the operating actuator such that the determined operating angle follows the computed target operating angle.
According to the constitution of the present invention, the operating angle is generated by the operation of the operating member in accordance with the load torque, which is sum of the control torque outputted by the operating actuator and the operating torque inputted by the driver. This control torque functions so as to do away with the deviation between the operating angle and the target operating angle. Accordingly, in a case in which the operating angle has not attained the target operating angle, the control torque serves as an auxiliary force for the operation of the operating member, and in a case in which the operating angle has exceeded the target operating angle, the control torque serves as a reactive force against the operation of the operating member.
The steering actuator is controlled such that the behavior index value follows the target behavior index value corresponding to the operating angle and load torque. The vehicle behavior index value comprising the yaw rate changes in accordance with the control of the steering actuator. The target operating angle corresponds to the behavior index value comprising the yaw rate, and the operating angle corresponds to the target behavior index value.
Accordingly, in a case in which the behavior index value has not attained the target behavior index value, since the operating angle exceeds the target operating angle, the above reactive force against the operation of the operating member functions. In accordance therewith, in a case in which the yaw rate does not increase due to a drop in the coefficient of friction between the surface of the road and the tires, or tire lateral force having reached its limit even when the operating torque is increased, the reactive force against the operation of the operating member can be made to function. Even if the driver increases operating torque at this time, the operating torque increase can be offset by the increase of this reactive force, and the load torque acting on the operating member can be maintained approximately constant, preventing an increase of the target behavior index value corresponding to the load torque and operating angle. That is, because the operating angle and load torque, and in turn, the target behavior index value can be held in check by this reactive force, the divergence of the steering angle can be prevented, and vehicle behavior can be stabilized. Further, in a case in which a delay occurs in the behavior index value following the target behavior index value due to a delay in the response of the steering actuator relative to an operation input, because the above reactive force functions, it is possible to alleviate the wrong feeling resulting from the delayed response of this steering actuator, thus enabling improved steering feel.
It is desirable that lateral acceleration and velocity are determined in addition to yaw rate as the above-mentioned behavior index value, that the target operating angle has a component corresponding to a value arrived at by dividing the lateral acceleration by vehicle velocity, and a component corresponding to the value of the yaw rate, and that the ratio of the component corresponding to the value of the yaw rate in the target operating angle changes in accordance with the vehicle velocity. Furthermore, it is desirable that this ratio increase in accordance with an increase in vehicle velocity.
In accordance therewith, it is possible to control the steering device with accommodating to vehicle behavior characteristics, such that the yaw rate becomes smaller at low velocity by making the affect of lateral acceleration greater at low vehicle velocity, and making the affect of the yaw rate greater pursuant to an increase in vehicle velocity, in response to the target operating angle corresponding to the behavior index value.
Furthermore, it is possible to control the steering device with accommodating even closer to vehicle behavior characteristics by making it possible to change setting value of the vehicle velocity, at the time when the component corresponding to a value arrived at by dividing the lateral acceleration by vehicle velocity is equal to the component corresponding to the value of the yaw rate in the target operating angle.
According to the present invention, in a vehicle that employs a steer by electric wire system, it is possible to provide a steering device, which prevents vehicle behavior from becoming unstable and steering feel from deteriorating, by controlling the torque acting on the operating member in accordance with vehicle behavior and setting a target behavior index value in accordance with the torque acting on the operating member.